Multilevel chain link converters are used in many high power applications. In particular, modular converters, where a number of switching cells, each including a number of switching elements, or switching units, and an energy storing element in the form of a DC capacitor, are connected in series in a chain link to form a variable voltage source, have found increased use. These modular converters are used in HVDC (High Voltage Direct Current) and FACTS (Flexible Alternating Current Transmission Systems) applications.
A commonly used modular converter consists of serially connected full-bridge switching cells, each switching cell comprising four switching units, in the form of semiconductor switches, for example IGBTs (Insulated-Gate Bipolar Transistor) or IGCTs (Integrated Gate-Commutated Thyristor), and one DC capacitor unit.
The losses in semiconductor switches is dependent on both the switching of, as well as the conduction by, the switches. In many FACTS converters, the conduction losses have a greater impact on total loss than the switching losses.
Semiconductor switches having higher voltages can be used in full-bridge converters to reduce the number of switching cells in order to reduce the conduction losses. However, reducing the number of switching levels reduces the available number of intermediate voltage levels. Thus, a trade-off will be made between harmonic performance of the converter and the switching frequency of the switches. In for example FACTS applications comprising full-bridge converters, it is difficult to reduce the number of switching cells beyond a certain point without negatively affecting the harmonic requirements so that the harmonic requirements of the power system cannot be met.
WO 2013/186006 describes an alternative to full-bridge switching cells, wherein the number of switching units are reduced for a converter having the equal number of switching levels. The number of switching units, for each capacitor of the converter, is half compared to the number of switches in a converter with full-bridge switching cells. For a five-level chain link, the number of switching units in the conduction path is three compared to four switching units in a full-bridge converter. Using the chain link described in WO 2013/186006, it is therefore possible to reduce the conduction losses in comparison to converters with full-bridge switching cells.
The paper “Five level cross connected cell for cascaded converters”, by Alireza Nami, Liwei Wang, Frans Dijkhuizen, presented at EPE, Lille, France, Sep. 3, 2013 (Nami et al) describes the same type of five level four quadrant multilevel converter cell configuration as in WO 2013/186006. Nami et al points out that a purpose of this cell configuration is to reduce costs and losses in a multilevel converter, by providing a large number of voltage levels with a low number of devices. The five-level four quadrant cell configuration of Nami et al is suitable for HVDC and FACTS converters and is depicted in FIG. 12. The switching cell has two half-bridges (the output switches S1, S2 and the input switches S3, S4, respectively), two switching units S5, S6, and two capacitors C1, C2, each cell half having one capacitor, wherein the cell halves are cross connected to each other, by means of the switching units S5, S6.
The chain link of Nami et al and WO 2013/186006 provides a way to reduce the number of switching units in the conduction path, and thereby reduce losses. However, there is still a need to reduce the losses even further, without lowering the quality in terms of harmonic performance of the chain-link converters